The present invention relates to diagnostic technique for EGR systems for recirculating part of exhaust gases of an engine.
A Japanese Patent Kokai Publication No. 5(1993)-5465 shows a conventional diagnostic system for detecting an abnormal condition in an EGR system.
It is an object of the present invention to provide a diagnostic system or process for accurately identifying and locating a cause of malfunction in an EGR system.
According to the present invention, an EGR system comprises:
an EGR valve varying an valve opening in accordance with an intake vacuum;
an auxiliary valve assisting the EGR valve;
a first parameter calculating section calculating an engine rotational fluctuation parameter in an EGR state in which an EGR operation is in progress;
a first checking section making a first decision that there is a possibility of a malfunction in the auxiliary valve causing an excess of an EGR quantity when the fluctuation parameter is greater than a first judgement value;
an EGR cutting section forcibly cutting off the EGR operation, irrespective of a condition for continuing the EGR operation, when the first decision is made;
a second parameter calculating section calculating the engine rotational fluctuation parameter in an EGR cutoff state in which the EGR operation is cut off; and
a second checking section making a second decision that there actually exists a malfunction in the auxiliary valve when the fluctuation parameter calculated by the second parameter calculating section is equal to or smaller than a second judgement value which is smaller than the first judgement value.
A diagnostic process according to the present invention comprises:
calculating an engine rotational fluctuation parameter in an EGR state in which an EGR operation is in progress;
making a first decision that there is a possibility of a malfunction in the auxiliary valve causing an excess of an EGR quantity when the fluctuation parameter is greater than a first judgement value;
forcibly cutting off the EGR operation, irrespective of a condition for continuing the EGR operation, when the first decision is made;
calculating the engine rotational fluctuation parameter in an EGR cutoff state in which the EGR operation is cut off; and
making a second decision that there actually exists a malfunction in the auxiliary valve when the fluctuation parameter calculated in the EGR cutoff state is equal to or smaller than a second judgement value which is smaller than the first judgement value.